An exhaust gas purifying catalyst is formed by coating a substrate made of ceramics or the like with a catalyst coating layer containing a refractory inorganic oxide and a noble metal. These days, most of catalysts are three-way catalysts that purify HC, CO and NOx at the same time.
One of properties required for an exhaust gas purifying catalyst is a property to be activated rapidly after engine start even in a low temperature range (i.e., ignition performance: low-temperature activation property). Researches have been performed to improve ignition performance of exhaust gas purifying catalysts, for example, by providing a catalyst layer with a two-layer structure (see Patent Documents 1 to 4).
For example, claim 1 of Publication of Japanese Patent No. 3235640 (Patent Document 1) recites that an inside layer of a catalyst layer contains a cerium-zirconium composite oxide with a weight ratio of 100/2 to 100/80 in terms of cerium/zirconium oxide, and an outside layer of the catalyst layer contains a cerium-zirconium composite oxide with a weight ratio of 2/100 to 100/100 in terms of cerium/zirconium oxide.
Claim 1 of Publication of Japanese Unexamined Patent Application No. 2004-298813 (Patent Document 2) recites that an inside layer of a catalyst layer is a composite ceramic containing platinum-supported alumina constituted by porous alumina supported with platinum component and an oxygen storing ceria-zirconia composite oxide, and an outside layer of the catalyst layer is a composite ceramic containing: at least one of rhodium supported ceria-zirconia composite oxide and rhodium supported alumina, each constituted by a low thermally degradable ceria-zirconia composite oxide or a porous alumina supported with rhodium component; and at least one of a porous alumina and a low thermally degradable ceria-zirconia composite oxide.
Claim 5 of Publication of Japanese Unexamined Patent Application No. 2001-70792 (Patent Document 3) recites that an inside layer of a catalyst layer contains palladium, and an outside layer contains both of a zirconium-based composite oxide coexistently supported with rhodium and platinum, and a cerium-based composite oxide coexistently supported with rhodium and platinum.
Claims 1 and 2 of Publication of Japanese Unexamined Patent Application No. 10-296085 (Patent Document 4) recites that an inside layer of a catalyst layer is constituted by a refractory inorganic oxide including active alumina as a main component, Ce, Zr, Pd, and Ba, and has a Ce/Zr mol ratio of 85/15 to 30/70. An outside layer of the catalyst layer is constituted by a refractory inorganic oxide including active alumina as a main component, at least one selected from Ce, Zr and their compounds, and Rh, and has a Ce/Zr mol ratio of 0/100 to 25/75.    Patent Document 1: Publication of Japanese Patent No. 3235640    Patent Document 2: Publication of Japanese Unexamined Patent Application No. 2004-298813    Patent Document 3: Publication of Japanese Unexamined Patent Application No. 2001-70792    Patent Document 4: Publication of Japanese Unexamined Patent Application No. 10-296085